// -*- mode: markdown;  markdown-fontify-code-block-default-mode: rustic-mode; evil-shift-width: 2; -*-
/*!

# My first `egg` 🐣

This tutorial is aimed at getting you up and running with `egg`,
  even if you have little Rust experience.
If you haven't heard about e-graphs, you may want to read the
[background tutorial](../_01_background/index.html).
If you do have prior Rust experience, you may want to skim around in this section.

## Getting started with Rust

[Rust](https://rust-lang.org)
is one of the reasons why `egg`
  is fast (systems programming + optimizing compiler) and flexible (generics and traits).

The Rust folks have put together a great, free [book](https://doc.rust-lang.org/stable/book/) for learning Rust,
  and there are a bunch of other fantastic resources collected on the
  ["Learn" page of the Rust site](https://www.rust-lang.org/learn).
This tutorial is no replacement for those,
  but instead it aims to get you up and running as fast as possible.

First,
  [install](https://www.rust-lang.org/tools/install) Rust
  and let's [create a project](https://doc.rust-lang.org/cargo/getting-started/first-steps.html)
  with `cargo`, Rust's package management and build tool: `cargo new my-first-egg`.[^lib]

[^lib]: By default `cargo` will create a binary project.
        If you are just getting starting with Rust,
          it might be easier to stick with a binary project,
          just put all your code in `main`, and use `cargo run`.
        Library projects (`cargo new --lib my-first-egg`)
          can be easier to build on once you want to start writing tests.

Now we can add `egg` as a project dependency by adding a line to `Cargo.toml`:
```toml
[dependencies]
egg = "0.6.0"
```

All of the code samples below work, but you'll have to `use` the relevant types.
You can just bring them all in with a `use egg::*;` at the top of your file.

## Now you're speaking my [`Language`]

[`EGraph`]s (and almost everything else in this crate) are
    parameterized over the [`Language`] given by the user.
While `egg` supports the ability easily create your own [`Language`],
  we will instead start with the provided [`SymbolLang`].

[`Language`] is a trait,
  and values of types that implement [`Language`] are e-nodes.
An e-node may have any number of children, which are [`Id`]s.
An [`Id`] is basically just a number that `egg` uses to coordinate what children
  an e-node is associated with.
In an [`EGraph`], e-node children refer to e-classes.
In a [`RecExpr`] (`egg`'s version of a plain old expression),
  e-node children refer to other e-nodes in that [`RecExpr`].

Most [`Language`]s, including [`SymbolLang`], can be parsed and pretty-printed.
That means that [`RecExpr`]s in those languages
  implement the [`FromStr`] and [`Display`] traits from the Rust standard library.
```
# use egg::*;
// Since parsing can return an error, `unwrap` just panics if the result doesn't return Ok
let my_expression: RecExpr<SymbolLang> = "(foo a b)".parse().unwrap();
println!("this is my expression {}", my_expression);

// let's try to create an e-node, but hmmm, what do I put as the children?
let my_enode = SymbolLang::new("bar", vec![]);
```

Some e-nodes are just constants and have no children (also called leaves).
But it's intentionally kind of awkward to create e-nodes with children in isolation,
  since you would have to add meaningless [`Id`]s as children.
The way to make meaningful [`Id`]s is by adding e-nodes to either an [`EGraph`] or a [`RecExpr`]:

```
# use egg::*;
let mut expr = RecExpr::default();
let a = expr.add(SymbolLang::leaf("a"));
let b = expr.add(SymbolLang::leaf("b"));
let foo = expr.add(SymbolLang::new("foo", vec![a, b]));

// we can do the same thing with an EGraph
let mut egraph: EGraph<SymbolLang, ()> = Default::default();
let a = egraph.add(SymbolLang::leaf("a"));
let b = egraph.add(SymbolLang::leaf("b"));
let foo = egraph.add(SymbolLang::new("foo", vec![a, b]));

// we can also add RecExprs to an egraph
let foo2 = egraph.add_expr(&expr);
// note that if you add the same thing to an e-graph twice, you'll get back equivalent Ids
assert_eq!(foo, foo2);
```

## Searching an [`EGraph`] with [`Pattern`]s

Now that we can add stuff to an [`EGraph`], let's see if we can find it.
We'll use a [`Pattern`], which implements the [`Searcher`] trait,
  to search the e-graph for matches:

```
# use egg::*;
// let's make an e-graph
let mut egraph: EGraph<SymbolLang, ()> = Default::default();
let a = egraph.add(SymbolLang::leaf("a"));
let b = egraph.add(SymbolLang::leaf("b"));
let foo = egraph.add(SymbolLang::new("foo", vec![a, b]));

// we can make Patterns by parsing, similar to RecExprs
// names preceded by ? are parsed as Pattern variables and will match anything
let pat: Pattern<SymbolLang> = "(foo ?x ?x)".parse().unwrap();

// since we use ?x twice, it must match the same thing,
// so this search will return nothing
let matches = pat.search(&egraph);
assert!(matches.is_empty());

egraph.union(a, b);
// recall that rebuild must be called to "see" the effects of unions
egraph.rebuild();

// now we can find a match since a = b
let matches = pat.search(&egraph);
assert!(!matches.is_empty())
```



## Using [`Runner`] to make an optimizer

Now that we can make [`Pattern`]s and work with [`RecExpr`]s, we can make an optimizer!
We'll use the [`rewrite!`] macro to easily create [`Rewrite`]s which consist of a name,
  left-hand pattern to search for,
  and right-hand pattern to apply.
From there we can use the [`Runner`] API to run `egg`'s equality saturation algorithm.
Finally, we can use an [`Extractor`] to get the best result.
```
use egg::{*, rewrite as rw};

let rules: &[Rewrite<SymbolLang, ()>] = &[
    rw!("commute-add"; "(+ ?x ?y)" => "(+ ?y ?x)"),
    rw!("commute-mul"; "(* ?x ?y)" => "(* ?y ?x)"),

    rw!("add-0"; "(+ ?x 0)" => "?x"),
    rw!("mul-0"; "(* ?x 0)" => "0"),
    rw!("mul-1"; "(* ?x 1)" => "?x"),
];

// While it may look like we are working with numbers,
// SymbolLang stores everything as strings.
// We can make our own Language later to work with other types.
let start = "(+ 0 (* 1 a))".parse().unwrap();

// That's it! We can run equality saturation now.
let runner = Runner::default().with_expr(&start).run(rules);

// Extractors can take a user-defined cost function,
// we'll use the egg-provided AstSize for now
let mut extractor = Extractor::new(&runner.egraph, AstSize);

// We want to extract the best expression represented in the
// same e-class as our initial expression, not from the whole e-graph.
// Luckily the runner stores the eclass Id where we put the initial expression.
let (best_cost, best_expr) = extractor.find_best(runner.roots[0]);

// we found the best thing, which is just "a" in this case
assert_eq!(best_expr, "a".parse().unwrap());
assert_eq!(best_cost, 1);
```

[`EGraph`]: ../../struct.EGraph.html
[`Id`]: ../../struct.Id.html
[`Language`]: ../../trait.Language.html
[`Searcher`]: ../../trait.Searcher.html
[`Pattern`]: ../../struct.Pattern.html
[`RecExpr`]: ../../struct.RecExpr.html
[`SymbolLang`]: ../../struct.SymbolLang.html
[`define_language!`]: ../../macro.define_language.html
[`rewrite!`]: ../../macro.rewrite.html
[`FromStr`]: https://doc.rust-lang.org/std/str/trait.FromStr.html
[`Display`]: https://doc.rust-lang.org/stable/std/fmt/trait.Display.html
[`Rewrite`]: ../../struct.Rewrite.html
[`Runner`]: ../../struct.Runner.html
[`Extractor`]: ../../struct.Extractor.html

*/